3rd Generation Partnership Project (3GPP) specifies interworking between 4th Generation (4G) communication networks and 5th Generation (5G) communication networks and also provides procedures in 5G Standalone (SA) architecture for enabling a 5G capable user equipment (UE) to handover from 5G New Radio (NR) to 4G Evolved UMTS Terrestrial Radio Access Network (EUTRAN) and vice-versa.
When a Protocol Data Unit (PDU) session is created on a 5G core for a UE, a User Plane Function (UPF) node is selected by a Session Management Function (SMF) to forward and process the data packets destined for and/or originating from the UE. During a handover of the UE from 5G to 4G EUTRAN, the Access and Mobility Management Function (AMF) node of the 5G network selects a 4G Mobility Management Entity (MME) node and initiates a context transfer to LTE core of the 4G EUTRAN network. As a part of this process, the MME node selects a Serving Gateway Control Plane (SGW-C) node and the SGW-C node in turn selects a Serving Gateway User Plane (SGW-U) node (in a Control and User Plane Separation of Evolved Packet Core nodes (CUPS) based LTE core). This SGW-C node then communicates with the Packet Gateway Control Plane (PGW-C) component of the SMF node to setup the LTE session.
Due to addition of SGW-U node in data path, the data to/from the UE in the 4G network now has one additional hop to travel (eNodeB→SGW-U→UPF→Data Network (DN) for uplink and DN→UPF→SGW-U→eNodeB for downlink). A typical user plane node can support functionalities of all SGW-U, PGW-U and UPF nodes. However in the currently defined 3GPP procedure for 5G to 4G handover, there is no mechanism for the SGW-C node to know whether the UPF node in the 5G core can also act as a SGW-U node in the 4G network for the UE. Also, SGW-C has no mechanism to know the identity of the UPF node selected by SMF node in 5G core. Hence, a SGW-C node, as of today, cannot guarantee selection of same user plane node to avoid the extra hop in data path as mentioned above. This lack of guarantee of the selection of the same user plane node as the SGW-U results in the above four-hop process, examples of which are visually depicted in FIGS. 1 and 2.
FIG. 1 illustrates an example of a four-hop process after a 5G to 4G handover of a UE. As noted above, the handover process currently specified by 3GPP does not guarantee that the same user plane node 102 used as the UPF node in the 5G network for UE 100 is again selected as the SGW-U for UE 100 when UE 100 is handed over to the 4G network (from the 5G plane to the 4G plane shown in FIG. 1). Instead, SGW-U 104 may have been selected. Therefore, any communication between UE 100 and DN 106 involves UE 100 communicating with eNB 108 (D1), eNB 108 communicating with the selected SGW-U 104 (D2), SGW-U 104 communicating with the old SGW-U+UPF/PGW-U node 102 (D3) and SGW-U+UPF/PGW-U node 102 communicating with DN 106 (D4). Therefore, a data path between UE 100 and DN 106 includes D1→D2→D3→D4.
FIG. 2 illustrates another example of a four-hop process after a 5G to 4G handover of a UE. Similar to FIG. 1, the handover process currently specified by 3GPP does not guarantee that the same user plane node 102 used as the UPF node in the 5G network for UE 100 is again selected as the SGW-U for UE 100 when handed over to the 4G network (from the 5G plane to the 4G plane shown in FIG. 1). Instead, SGW-U 200 may have been selected. Therefore, any communication between UE 100 and DN 106 involves UE 100 communicating with eNB 108 (D1), eNB 108 communicating with the selected SGW-U 200 (D2), SGW-U 200 communicating with the old SGW-U+UPF/PGW-U node 102 (D3) and SGW-U+UPF/PGW-U node 102 communicating with DN 106 (D4). Therefore, a data path between UE 100 and DN 106 includes D1→D2→D3→D4.
This four-hop process in the data path can also add to latency and may impact the low latency requirement for 5G applications.